I've never actually used it, but I would mix it with some corrugated cardboard (box material) and maybe run sume water through it before I used it. I think it would be fine but just to be safe I would probably let it age for a month or so. It does have some salt in it, but that is a result of the manure used to make it. Here is a link that might be helpful if you wanted to read a little more about it:

Here is a link that might be useful: musroom spawn

I have used wood pellets. When wet they expand into sawdust. I used to put the pellets under my bin to soak up the leachate. It would also house any escaped worms. After it had been wet for a couple weeks i transferred it to the top of my worm bin. The worms never had any problems with it.

But you do have to be careful that the wood is not cedar. It has oils that can harm the worms.

mushroom substrate is probably going to be awfully acidic and contain lots of proteins. The only problem I can see is potential protein poisoning. If your worms start looking mutated and falling apart, that's it. I'd at least add calcium. Why not start a new bin and experiment? Just transfer some worms over. Or divide the existing bin and use the substrate in half and see if the worms go for it. If they don't, there is likely a good reason.